Flow and heat transfer in gas-liquid slug flow in small diameter channels have been studied extensively in the last few decades because of its unique ability to segment the flow and enhance heat transfer by the internal recirculation in the liquid phase. The segmentation of the continuous liquid phase is achieved by the gas bubble of the size of the channel. While the hydrodynamics and heat transfer for long Taylor bubbles having volume more than that of a sphere that can fit in the channel has been studied extensively, very little attention has been paid to the bubbles having smaller volume but almost spanning the channel. The bubble volume can be represented by a non-dimensional equivalent sphere radius, ratio of the radius of a sphere having same volume as that of the bubble and channel radius. In this work, we study the hydrodynamics of the slug flow for a range of bubble volumes keeping all other parameters constant for non-dimensional equivalent sphere radius close to 1, between 0.72 and 1.55. The bubble shape, pressure distribution, bubble velocity and flow field in the liquid slug has been investigated. The effect of Reynolds number on the bubble shape for short as well as Taylor bubbles has also been studied. Heat transfer without phase change for constant heat flux boundary condition at the wall has been investigated and the Nusselt number is found to be highest for the non-dimensional equivalent sphere radius close to one. The heat transfer results have also been compared with a simple phenomenological model available in literature for heat transfer in slug flow. (C) 2018 Elsevier Ltd. All rights reserved.